. . . Testing for Zika infection is becoming more difficult, making it harder for doctors to advise pregnant women about the chances their child might have a Zika-related birth defect, the Centers for Disease Control and Prevention revealed in a health advisory issued Friday.

The CDC is now suggesting that women thinking of getting pregnant, and who may be exposed to the Zika virus through travel or because of where they live, should consider having their blood tested for Zika antibodies before they get pregnant. Having a baseline reading would help to interpret Zika tests done during a later pregnancy.